The present disclosure relates to a system and method for mapping a target scene using scanning radar utilizing the Doppler effect that arises in the event of movement between the radar and the target scene, in which the movement of a platform on which the radar's antenna is mounted is calculated utilizing navigation data obtained for the platform.
As radar is one of the few available sensors for detailed ground mapping, there are continual requirements for further development of the technology. Other commonly used sensors, such as infrared and video sensors, utilize only image processing for image analysis, whereas, using radar technology, it is also possible to take advantage of the signal characteristics that are unique for each specific target. The radar technology has thus the advantage that signal processing and image processing can be combined.
Viewed historically, radar has been of great significance in association with military applications. At its commencement, the technology made possible the detection of aircraft and vessels. In spite of the limitations of the systems of the time, the enemy could usually be detected in good time, whereby unnecessary losses were avoided. Today, thanks to the developments in technology, detection capabilities have improved considerably. As modern radar technology, in combination with complex signal and image processing, in many cases enables radar images to be of photographic quality, reconnaissance over land and inside archipelagos is nowadays a normal radar application.
In spite of the developments, problems still remain that restrict the use of radar. One such a problem relates to the ability to generate high-resolution radar images within an adjacent angular interval around the platform motion direction. Phenomena that limit the image generation include Doppler variation and range or distance variation. Both phenomena will be discussed in greater detail below with reference to the figures. A situation as described above with forward-scanning radar is very common in association with military applications, where an approach is expected to take place in the direction of the target.
Ever since the principles of Doppler resolution became known, radar engineers have tried to utilize in an optimal way the Doppler effect that arises when there is movement between radar and target scene. It will be demonstrated below that the Doppler bandwidth is of decisive importance for the size of the angular resolution. As the illumination angle, that is the angle between movement vector and target, has a large influence on the Doppler characteristics of the illuminated target, the angular resolution is also dependent upon a corresponding angle. Angular resolution, that is given by effective antenna beam width ψe divided by a predefined beam sharpening factor RFSAR, is derived below according to
      ψ    d    =                    ψ        e                    R        FSAR              =                            λ          /          l                                                    2              ⁢                              λυ                p                                                                    ω                s                            ⁢                              l                2                                              ⁢                      sin            ⁡                          (              ϕ              )                                          =                                    ω            s                    ⁢          l                          2          ⁢                      υ            p                    ⁢                      sin            ⁡                          (              ϕ              )                                          
where λ corresponds to the signal's wavelength, l is the physical antenna size, νp is the platform speed, ωs is the antenna's scan rate and φ is the antenna angle.
At a constant scan rate, all parameters apart from sin(φ) can be assumed to be constant. As the term sin(φ) is found in the denominator in the correlation above, it can be seen that optimal resolution is obtained for the target angle 90°, while small target angles (→0°) do not allow any coherent integration. The angle 0° corresponds here to the platform's direction of travel, while the angle 90° corresponds to an antenna angle perpendicular to the direction of travel.
In a target seeker application, an angular interval of approximate size±30° is of particularly great interest, as the approach is assumed to be taking place towards a threatening object.
Traditionally, radar modes that utilized forward-scanning antenna have been classified within the category DBS (Doppler Beam Sharpening), see Donald R. Wehner, “High-Resolution Radar, Second Edition”, ISBN 0-89006-727-9, Artech House 1995.
The focusing that traditionally was carried out by filtering, often required extremely complex filter banks to be applied. As each subfilter was optimized for a given spectral area (regarding band width and sidelobe handling), a large number of subfilters were required in order to cover the whole spectral area.
As modern spectral analysis increasingly utilizes FFT-based (Fast Fourier Transform) tools, these methods have increasingly replaced old technology. Utilizing FFT-related methods, traditional bandpass filtering can be carried out and, in addition, more precisely matched filtering is made possible. The methods differ in execution and also in how the received signal quantity is to be pre-processed. Focusing by bandpass filtering requires a frequency-expanded signal in order for focusing to be achieved. Matched filtering in turn requires a demodulated signal quantity where angle-separated targets are distinguished by frequency.
Matched filtering integrates all the signal energy belonging to a particular frequency component (a particular target).
Bandpass filtering suppresses unwanted frequency components.